This invention relates to the field of processes for cleaning the internal surfaces of organically contaminated large, closed-vessel pieces of equipment (i.e., distillation vessels) and other support equipment (that can be isolated with steam and water either individually or collectively in closed “circuits”) located in refineries and other petrochemical plants.
Common to the refining industry, a “turnaround” is the process of taking single or multiple distillation vessels off-line for maintenance and/or inspection. Multiple maintenance applications are performed during this time, including the replacement of valves, pipes, trays, spargers, packed sections, boilers, exchangers, and other components.
A “squat,” which is a limited, less time-consuming version of a turnaround, usually involves taking only part of a pipestill section off-line (i.e., the vacuum vessel but not the atmospheric vessel).
A turnaround is performed for several reasons, some of which are mandated by the federal government and others determined by refinery operational needs. The government requires inspections on distillation vessels for safety reasons. In addition to mandated inspections, the refinery also may take a pipestill section, or a particular distillation vessel, off-line if it believes that the pipestill performance will be improved by modifying existing equipment or by performing planned or unplanned maintenance.
Thus, a turnaround is an infrequent opportunity for the refinery operator to enhance the performance of the vessel(s), thus increasing overall efficiency of the pipestill section. Processes in the refinery are intimately connected, thus deficiencies or enhancements in pipestills can significantly affect downstream applications and costs.
The timing of a turnaround, and the amount of time that the pipestill section or vessels are off-line, is very critical to the profitability of a refinery. As in other continuous process industries where demand for the product is also continuous, idle equipment causes an irreversible loss of revenue. In the case of a refinery, one day lost in production may cause several millions of dollars to be lost in revenue. Because of this, refineries will spend several months planning every step of the turnaround process in order that it is done quickly, safely, and efficiently. A reduction of days, or even hours, from the turnaround process gains the refinery significant marginal income.
During a turnaround, and before internal mechanical maintenance is performed of any kind, a cleaning must take place which frees all the internal surfaces of the refinery components from contaminants. These internal surfaces may include the walls of the vessel cylinder, the tops and bottoms of trays, packing sections (loose or fixed), spargers, pump-around piping, and especially the bottom third of the vessel. The bottom section is typically very difficult to clean since it is the area that produces the heavier factions of hydrocarbons. The quicker this cleaning is accomplished, the sooner industry-cleanliness standards will have been met. Until then, however, workers will not be permitted entry into the vessel.
The contaminants removed would include any hydrocarbon that is found in crude oil. These hydrocarbons will vary in size, length, molecular weight and structure. The industry refers to these different structures as Light End, Medium and Heavy. Light Ends would be cuts like methane, propane, ethane, and the like. Medium cuts would include kerosene, gasoline, and diesel, among others. Heavy cuts would include lubricants, waxes and asphalt.
There are several reasons why distillation vessels and other supporting equipment must be effectively cleaned before interior maintenance is performed.
A first reason involves the removal of dangerous fumes. If the hydrocarbons are not effectively cleaned from the vessel, an accumulation of by-product fumes (i.e., H2S gas) will remain therein. These gases are deadly to humans—especially when that exposure occurs within a confined space. By federal law, refinery operators must reduce hydrocarbon levels below industry maximums before allowing people to enter the vessel to perform work. If levels are not low enough upon reading, the vessel must either be recleaned or vented to the atmosphere for hours or days.
A second reason involves reduction of fire hazards. It is not uncommon for welders to accidentally set vessels on fire during mechanical work if the vessels are not cleaned thoroughly. This level of cleanliness is especially important in the packed sections of a vessel which may trap significant hydrocarbons, causing high LEL readings upon entry if not properly cleaned. Therefore, the refinery components must be thoroughly cleaned to prevent the danger of fire.
A third reason involves enabling more effective visual inspections. It takes operators and federal inspectors longer to inspect a vessel, if that vessel is not properly cleaned. This is because inspectors are looking for fatigue or cracks in the trays or walls along with other potential signs of failure. If the potential exists that defects may be hidden by unremoved contaminants, it will take the searcher longer to determine whether or not such defects exist. Thus, the process is made more time-consuming and costly.
A fourth reason involves overall safety. Quite simply, the potential for slips, falls and other mishaps in the vessel are reduced when the metal is freed from oils, waxes and greases. Therefore, thorough cleaning reduces the likelihood of injury to workers.
A fifth reason involves process efficiency. When a process vessel is contaminated, pressure drops occur which limit the process throughput or output rates. When the contaminant is removed, flow rates may be increased, with a subsequent improvement in operating efficiency.
There are several known methods for cleaning pressurized vessels in petroleum refineries known in the prior art.
One such method involves basic steam cleaning. With this method, the refinery first takes the pipestill off-line. Reduced crude (gas oil) is then circulated through the vessel. Reduced crude or gas oil is primarily medium to light end hydrocarbons similar to kerosene. The reduced crude physically displaces solid materials from the vessel, and takes approximately 48 hours to complete. After the reduced crude wash, the vessel is completely emptied. High-pressure steam is then piped into the vessel. While simple and relatively inexpensive, the cleaning performance of high-pressure steam is very poor. Refineries will steam a vessel in this manner for as long as five days. After the steaming process, the vessel must be tested for hydrocarbon gas. Workers can not enter the vessel until the hydrocarbon gas levels have been reduced to a safe level. By itself, the steam process will not reduce the hydrocarbon gas. Therefore, the vessel is usually opened to the atmosphere until the hydrocarbon gas has volatilized and moved out of the vessel. This airing out process may take as long as two days before entry is gained to the vessel. Once the hydrocarbon gas level has been reduced to an acceptable level, a cleaning crew may be sent in. The cleaning crew usually comprises four to six workers. These workers, once inside the vessel, physically mop or scrape components until they are clean. This process may take the crew an average of four to six days. Once the vessel has been cleaned, welding, maintenance, and repair can begin.
Another method incorporates liquid cleaning with a caustic solution. Caustic solution cleaning begins like the basic steam cleaning method—with a reduced crude wash. After the reduced crude wash, caustic or high-pH chemicals are circulated through the vessel. The caustic chemicals are usually diluted with water and circulated through the vessel in the liquid phase. Circulating the caustic chemicals in the liquid phase requires a high volume of liquid to reach the entire surface area in the vessel. The liquid circulation process will normally last 48 hours. After the caustic chemical wash, the vessel is drained. Effluent collected from the caustic chemical wash must be collected and treated. Due to the high pH of the caustic chemical, effluent generated during the caustic wash must be neutralized with an acid to neutralize the pH before the significant quantities of effluent are sent to a wastewater-treatment plant for processing. Additional processing may be required if the caustic chemicals contain phosphates, silicates, or other chelating agents that can interfere with the waste-treatment process. Just like the basic steam cleaning method, the vessel is opened to the atmosphere for up to two days to volatilize out any remaining hydrocarbons before crews may enter to mop and scrape the interior surfaces.
Yet another method involves an organic solvent wash. This method, like the first two, begins with a reduced crude wash. Next, organic solvents are circulated through the vessel from top to bottom. Although these organic solvents may satisfactorily remove oils, they do not have the solvency strength necessary to thoroughly clean the vessels while in a liquid phase. Solvent circulation can last as long as 24-48 hours. After the liquid phase cleaning, a water rinse is used to remove organic contamination from the vessel. Since organics by nature are not water soluble, rinsing with water is time-consuming, inefficient, and very difficult. Additionally, it is extremely difficult to determine whether these potentially harmful organics have been completely removed by the rinse process. Just like with the earlier methods, the vessel is exposed to the atmosphere to volatize out any remaining hydrocarbons, and a cleaning crew is then sent into the vessel to mop and scrape. Many times, failure to clean all surfaces results in high noxious gas readings (H2S, Benzene etc.) causing workers to don “fresh air” breathing apparatus. This apparatus slows down the turnaround and subjects workers to the hazardous environment. Yet another, and perhaps the greatest disadvantage of cleaning distillation vessels using a liquid phase procedure is the inability to get the underneath side of the equipment clean. Distillation trays, packed sections, and pall rings need to be cleaned on all sides before hot work can begin. Because these areas cannot be reached by the organic solvent wash, and because contaminants on these surfaces raise the possibility of noxious gas creation, and preclude inspection and maintenance activities the refiner is required to manually clean the tray bottoms, a process that is difficult, time consuming and dangerous.
In summary, each of these prior art methods incorporate individual processes that are particularly time-consuming and largely ineffective. What's more, additional time is consumed by the requirement that the vessel be exposed to the atmosphere to remove harmful gas and then manually cleaned to remove contamination.
The present invention overcomes these disadvantages in the prior art methods by introducing a cleaning agent in small specifically regulated quantities into vessels (and/or supporting equipment) by the use of steam. The steam volatilizes the cleaning agent and quickly dissolves the organic residues from the vessel. The cleaning agent used is comprised of a terpene and surfactant.
Terpenes have been used in refineries before. A liquid-steam method using terpenes is disclosed in U.S. Pat. No. 5,356,482 (“the '482”). The methods disclosed in the '482, however, are much different than those disclosed here. First, the '482 discloses the use of terpenes to detoxify the insides of a component in a refinery to remove dangerous and explosive gases. The method of the present invention, however, is directed to a technique of cleaning (or degreasing) the metal surfaces inside the refinery component—cleaning that component of essentially all contaminants on its interior surfaces. Not just degassing or masking/coating remaining hydrocarbons.
Second, the '482 suggests the use of recirculation for cleaning larger vessels such as fractionation towers, whereas circulation is specifically not a part of the process of the present invention. In fact, recirculation, if employed as part of the present invention would simply recontaminate many internal surfaces within the tower. The process of the present invention has been shown to work well for degassing and cleaning without circulation.
Third, the '482 methods further require the vessel to be completely sealed under pressure and to cool—a technique that has been known to occasionally cause catastrophic collapse. After a first cleaning, the insides of the vessel are sampled for the presence of noxious gas. The process of cleaning-cooling-sampling is repeated until a particular sampling shows that noxious gas is reduced to acceptable levels. This iterative process is unnecessarily time consuming and potentially hazardous to the people performing the process by comparison to the present invention.
The process of the present invention, however, requires instead that the equipment be ventilated either to atmosphere or to subsequent equipment in the refinery as part of a cleaning “circuit.” Additionally, contaminant is removed through the addition of a predefined amount of chemical rather than by sampling and process repetition.